This invention relates generally to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
An electrostatographic printing process forms an electrostatic latent image and reproduces the image in viewable form on a copy sheet. The field of electrostatography includes electrophotography and electrography. Electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, the electrostatic latent image. Electrography utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent image. In both of the foregoing processes, the latent image is rendered viewable by the process of developing, i.e. depositing particles thereon. Frequently, the particles are transferred from the latent image to a copy sheet. In some processes, the recording sheet on which the latent image is produced, may also serve as the copy sheet after the particles have been deposited thereon. However, in either case, the resultant toner powder image deposited on the sheet is permanently affixed thereto by applying heat and/or pressure. Hereinafter, an electrophotographic printing machine will be described as an illustrative embodiment of these processes.
In electrophotographic printing, the photoconductive member is charged to sensitize its surface. The charged photoconductive member is exposed to a light image of the original document being reproduced. Exposure of the sensitized photoconductive surface discharges the charge selectively. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document being reproduced. Development of the electrostatic latent image recorded on the photoconductive surface is achieved by bringing developer material into contact therewith. Typical developer materials comprise dyed or colored heat settable plastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. Thus, when developer material is brought into contact with the latent image recorded on the photoconductive surface, the greater attractive force thereof causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image. This concept was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the art.
Various methods have been developed for applying the developer material to the latent image. For example, the developer material may be cascaded over the latent image and the toner particles attracted from the carrier granules thereto. Other apparatus employed to develop the latent image include the use of magnetic field producing devices which form brush-like tufts extending outwardly therefrom contacting the photoconductive surface.
With the advent of single component developer i.e. conductive particles, carrier granules are no longer required. U.S. Pat. No. 2,846,333 issued to Wilson in 1956 describes a typical single component developer material. In general, the charged particles employed with the single component developer material have low resistivities which range from about 10.sup.4 to about 10.sup.9 ohm-cm. These particles are developed on the latent image. However, when the particles are transferred from the photoconductive surface to the copy sheet, repulsion frequently occurs. Transfer is optimized by employing particles having higher resistivities. Contrawise, development is optimized by utilizing particles having low resistivity or good conductivity. Thus, the system is faced with two contradictory requirements, i.e. particles having low resistivity for optimum development and high resistivity for optimum transfer.
Accordingly, it is a primary object of this invention to improve the development system employed in an electrophotographic printing machine so as to optimize both development and transfer.